Non-hematopoietic progenitor cells that reside in the body and give rise to multipotential cells when isolated are referred to as Mesenchymal Precursor Cells (MPCs). More specifically, purified MPCs are capable of forming very large numbers of multipotential cell colonies.
Simmons et al. (1994) describes enrichment of MPCs from freshly harvested bone marrow cells by selecting for cells that express the STRO-1 cell surface marker. As explained by the authors at pages 272-273, it is known that bone marrow cells contain a proportion of MPCs that are capable of giving rise to CFU-F. These CFU-F in turn are capable of giving rise under appropriate conditions to a broad spectrum of fully differentiated connective tissue, including cartilage, bone, adipose tissue, fibrous tissue and myelosupportive stroma.
MPCs and CFU-F are typically present at a very low incidence in bone marrow cells (typically between 0.05%-0.001%) and this rarity has been a major limitation to their study in the past. An important finding discussed by Simmons et al. (1994) was the identification that these MPCs could be enriched from freshly isolated bone marrow cells to some extent by selecting for STRO-1 positive cells. In particular, the selection of STRO-1 positive cells enabled isolation of MPCs (and resultant CFU-F) free of contaminating hemopoietic progenitors.
WO 01/04268 provided a further important advance in the enrichment of MPCs by identifying a subpopulation within this fraction of STRO-1 positive cells that contains MPCs. In particular, WO 01/04268 describes the sorting of the STRO-1 positive cell population into three subsets: STRO-1dull, STRO-1intermediate and STRO-1bright. Clonogenic assays for CFU-F in the different sorted subpopulations demonstrated that the vast majority of the MPCs are contained within the STRO-1bright fraction.
WO 2004/085630 discloses for the first time that MPCs are present in perivascular tissue. One of the benefits of this finding is that it greatly expands the range of source tissues from which MPCs can be isolated or enriched and there is no longer an effective restriction on the source of MPCs to bone marrow. The tissues from which MPCs can be isolated according to the methods described in WO 2004/085630 include human bone marrow, dental pulp, adipose tissue, skin, spleen, pancreas, brain, kidney, liver and heart. The MPCs isolated from perivascular tissue are positive for the cell surface marker 3G5. They can therefore be isolated by enriching for cells carrying the 3G5 marker, or by enriching for an early developmental surface marker present on perivascular cells such as CD146 (MUC18), VCAM-1, or by enriching for high level expression of the cell surface marker STRO-1.
The avascular connective tissues are generally located at anatomical sites within the musculoskeletal system that require appreciable movement. These freely movable joints are responsible for the majority of articulations in mammals. In synovial joints the contact surfaces of two opposing bones are covered by hyaline cartilages which glide effortlessly over each other because of the presence of a low friction lubricant in synovial fluid produced by the cells lining the joint capsule which overlays and connects the long bones. In the spinal column articulation is achieved by connection of the rigid vertebral bones by means of a flexible fibrocartilagenous ring (the annulus fibrosus) that encapsulates a hydrated gelatinous mass (the nucleus pulposus), populated by chondrocyte like cells similar to those present in hyaline cartilage. Irrespective of the type and location of these avascular connective tissue they all contain cells which synthesise an extracellular matrix which is rich in highly negatively charged proteoglycans, which imbibe water molecules together with the fibrous protein, type II collagen, which confers high tensile strength.
Avascular connective tissues such as hyaline cartilage, the inner two thirds of the meniscus and the intervertebral disc have limited repair capabilities and when injured may respond by the production of a functionally inferior fibrocartilagenous scar tissue. Through a multitude of factors, dominated by aging, genetics, hormonal status and physical injury these avascular connectives often fail leading to the widespread clinical problems of disc degeneration, back pain and osteoarthritis.
Current medical therapies normally used to treat the symptoms arising from the failure of these connective tissues, for the most part, do little to redress the underlying pathology responsible for producing the symptoms and in many instances may even exacerbate the problem by down regulating the capacity of the resident cells to synthesis the structural components of the tissue extracellular matrix. Ideally, therapeutic treatments should be at least chondroprotective but even provide the conditions which enhance matrix biosynthesis and effect repair and restoration of the injured connective tissues.